This invention relates generally to the NTSC color television system which carries picture information by means of luminance and chrominance signals, and more particularly to a system for pre-filtering the luminance and chrominance signals prior to encoding of the NTSC color television signal.
It is well known that the NTSC color encoded signal consists of a luminance signal and a chrominance signal, the latter, in turn, consisting of two color difference signals modulating, in quadrature, the NTSC color subcarrier. The chrominance signal shares with the luminance signal the frequency band from approximately 2.1 MHz to 4.2 MHz. It is also known that, in general, the spectral energy of the luminance signal is concentrated at harmonics of the television horizontal scan line frequency, whereas the chrominance spectral energy is concentrated at odd harmonics of one-half the horizontal scan line frequency. This implies that, in general, within the spectral band occupied by both luminance and chrominance, the luminance signal energy and the chrominance signal energy are spectrally interleaved. This spectral interleaving permits the use in a color decoder of filters of the interleaving type, called comb filters, to nearly completely separate the luminance and the chrominance signals. However, because spectral interleaving of the luminance and chrominance does not occur at sharp luminance or chrominance transitions along or in the direction of the vertical picture axis, there is a certain amount of luminance/chrominance crosstalk which results in the luminance being mistakenly decoded as color signals and/or the chrominance being displayed as a luminance signal. It has been known for a decade or more that the crosstalk problem can be reduced by pre-filtering the luminance present within the frequency band shared with the chrominance, and also prefiltering the chrominance, prior to encoding the signals into an NTSC color signal. Such pre-filtering serves to reduce or eliminate those vertical signal transitions that cause the luminance to spread into the chrominance spectral gaps, and vice versa, and has heretofore been achieved by comb filtering or anti-alias filtering. Pre-comb filtering of both the chrominance signal and the luminance signal present within the spectral frequency band of the chrominance not only reduces cross-luminance and cross-color defects, but also reduces interline flicker of high frequency details, a known defect of interlaced television systems.
Such known pre-comb filtering systems are not without defects, however, in that combing of the chrominance signal within the chrominance band may introduce errors that are extremely difficult to avoid. For example, the chrominance signal normally is bandpass filtered prior to comb filtering; however, the bandpass filter must have a perfectly linear phase response characteristic if there is to be perfect combing, which, is very difficult to obtain and maintain. Further, because the chrominance signal is on a subcarrier and has a partial vestigial sideband, phase relationships and distortions at the upper or lower chrominance sidebands results in a less than perfect combing of that chrominance signal. Accordingly it would be desirable to be able to comb filter the chrominance signal without incurring distortions and errors due to the modulation and bandpass filtering processes.
Another problem inherent in interlaced scanning systems known as low frequency interline flicker is often caused by the signal attempting to convey excessive vertical resolution, and cannot be alleviated by comb filtering the chrominance and the higher frequency luminance signals. The low frequency interline flicker is a visual indication that the Nyquist limit is being violated along the vertical picture axis. Although an interlaced system is quite forgiving of such violation, so long as they are not excessive, some signal sources generate very high vertical spatial frequencies which, in an interlaced system, will cause highly visible low frequency interline flicker. This is particularly true of computer generated signals and certain signals from charge coupled pick-up devices (CCD). Therefore, there is a need for apparatus to, at times, pre-filter the luminance signal along the vertical scan axis at frequencies residing in the spectral frequency band below the chrominance spectral frequency band.
One object of the present invention is to provide an improved system for comb filtering the chrominance signal which circumvents the problems associated with comb filtering a signal that is modulated on a subcarrier and has undergone bandpass filtering.
Another object of the present invention is to provide a system for pre-filtering a luminance signal along the vertical picture axis so as to reduce the visibility of low frequency interline flicker, and which also provides conventional higher frequency luminance comb filtering.